A process for the preparation of catalyst precursors, and catalysts made therefrom useful for conducting carbon monoxide hydrogenation reactions, especially Fischer-Tropsch reactions, and products made from such catalysts. It also relates to a process for producing, reactivating and/or increasing the activity of catalysts for use in conducting such reactions.
Processes for the hydrogenation of carbon monoxide to produce waxy and/or oxygenated products for upgrading to highly valued chemical materials and/or hydrocarbon fuels are well documented in the technical and patent literature. For example, in the Fischer-Tropsch (F-T) process, it is well known that the carbon monoxide component of synthesis gas can be catalytically converted by reaction with the hydrogen to reduction products constituting a range of waxy liquid hydrocarbons; hydrocarbons which can be readily upgraded to transportation fuels. In these processes, e.g., catalysts constituted of Group VIII metals (Periodic Table of the Elements, Sargent-Welch Scientific Company, Copyright 1968), notably the Iron Group metals, particularly iron, ruthenium and cobalt, are generally preferred for the synthesis of C5+ hydrocarbons; and copper has become the catalytic metal of choice for alcohol synthesis. These metals can exist in multiple valence states, and each state can display quite different behavior from the others during the reduction treatments, hence impacting the catalytic properties of the active catalyst. Each of the metals can be promoted or modified with an additional metal, or metals, or oxide thereof, to improve, e.g., the activity and/or selectivity of the catalyst in conducting these reactions.
Iron Group metal surfaces exhibit higher activities for catalytic reactions such as hydrogenation, methanation and F-T synthesis when catalysts on which these metals are dispersed are subjected to high temperature oxidation, and subsequent reduction. Recent art can be found in Applied Catalysis, A, General 175 (1998) pp. 113-120 and references therein. U.S. Pat. Nos. 4,492,774; 4,399,234; 4,493,905; 4,585,789; 4,088,671; 4,605,679; 4,670,414 and EPO 253924 disclose activation of cobalt catalysts by means of a reduction/oxidation/reduction (R-O-R) cycle, resulting in an increase in activity for F-T synthesis. Thus, typically such catalyst, e.g., supported reduced Co in the form of either a freshly prepared catalyst, or a low activity or deactivated catalyst, is contacted at high temperatures ranging from about 300xc2x0 to about 600xc2x0 C. with a gaseous oxygen-containing stream to oxidize the metal, or metals, to a metal oxide form, e.g., Co3O4. Precautions are taken during such treatments to control the exothermicity of the reaction to avoid sintering of the oxide metal particles, an effect which can be detrimental to the activity of the catalyst. On reduction, i.e., on completion of the oxidation-reduction cycle, the dispersed oxide particles, e.g., the Co3O4, of the catalyst are reduced to dispersed metallic particles and the activity is increased or the fresh catalyst activated.
Considerable progress has been made in the development of catalysts, and processes, these providing good activity, and selectivity in alcohol synthesis, and in the conversion of hydrogen and carbon monoxide to distillate fuels, predominantly C5+ linear paraffins and olefins, with low concentrations of oxygenates. Nonetheless, there remains a pressing need for improved catalysts, and processes; particularly processes for activating and regenerating such catalysts.
This need and others are achieved in accordance with the present invention relating to an Aqueous Low Temperature Oxidation (ALTO) process for the preparation of, or the activation or reactivation of a cobalt catalyst. The process requires, beginning with a cobalt catalyst or catalyst precursor constituted of a composite having a particulate solid support, or powder component, and a metal, or metals component, inclusive of cobalt at least a portion of which is present as metallic cobalt, catalytically active for conducting carbon monoxide hydrogenation reactions, especially F-T synthesis reactions, contacting the catalyst or catalyst precursor with an oxidant in the presence of water, at sufficiently low temperature to avoid complete evaporation of the water. On reduction, the hydrogenation activity of the cobalt catalyst or catalyst precursor is increased. The hydrogenation activity, especially the carbon monoxide hydrogenation activity of the catalyst precursor, is increased by oxidizing and converting at least initially all or a portion of the metal, or metals component of the catalyst or catalyst precursor to cationic or oxy-anionic species. By oxidation, as used in the ALTO process is meant the conversion of a catalytic metal species to an ionic state, e.g., the conversion of the Coo species to a Co2+ species.
The ALTO process can be effected by contacting the catalyst or catalyst precursor in the presence of water at reaction conditions with gaseous oxidants or by using water-soluble or water miscible oxidants. Typically, the aqueous oxidizing liquid is water to which an oxidant is added. On contact of the catalyst or catalyst precursor with the aqueous oxidizing liquid, it is found that the catalytic metal(s) component thereof, at least a portion of that which is present as metallic cobalt, is transformed to a multitude of metal salts or compounds, including but not limited to metal hydroxides, nitrates, metal oxy-anions, oxyhydroxides, and the like. On reduction of the oxidized metal species, as may be produced by contact and treatment of the oxidized catalyst or catalyst precursor with hydrogen or a hydrogen containing gas, the oxidized metal, or metals is reduced to metallic metal or metals, e.g., Co; and the catalyst or regenerated catalyst thereby activated. Optionally, the oxidized catalyst or catalyst precursor may be dried. The metal compounds or salts formed during drying may be converted to metal oxides, i.e., CoO or Co3O4. In yet another option, the oxidized catalyst or catalyst precursor may be dried and calcined in an oxidizing atmosphere to obtain yet another oxidized catalyst precursor containing metal oxide or metal oxides, e.g., Co3O4. In both options, the catalyst or regenerated catalyst is activated by reduction of the oxidized catalysts or catalyst precursors. The catalyst precursors made from the catalyst or catalyst precursor via aqueous oxidation are useful for improving the performance, e.g., activity or selectivity of the activated or regenerated catalysts in carbon monoxide hydrogenation, especially F-T synthesis reactions.
The exothermicity of the ALTO treatment is effectively controlled by the high heat capacity and high heat transfer properties of a sufficient volume of liquid water. The reaction may be carried out at incipient wetness conditions or with an excess of water, resulting in a catalyst/water slurry. Upon completion of the reaction, excess water is eliminated by either filtration, decantation or by evaporation. By excess water is meant any water in excess of the volume necessary for filling the pore volume of the oxidized catalyst.
During the oxidation treatment, other metal salts or metal oxy-anion derivatives, e.g., perrhenic acid or salts, may be formed. When such salts or oxo-anions derivatives are soluble, it is preferable to use the evaporation or the incipient wetness methods in order to retain such metal salts on the catalyst surface. On completion of the oxidation treatment, the oxidized catalyst is then dried.
The metal compounds or salts as well as any metal oxy-anion(s) are intimately dispersed on the surface of the support, hence providing upon reduction with hydrogen or a hydrogen containing gas, metal (or metals) crystallites which are highly active species in carbon monoxide hydrogenation. Optionally, when the oxidized catalyst or catalyst precursor is calcined, the metal compounds or salts are converted to oxide particles without the deleterious effect of the intense exothermic reaction of directly converting a reduced metal in the higher valence oxide, Co3O4. The high performance of catalysts made by this process, and the fact that the ALTO step can be carried out at low temperature, are consequences of considerable importance in the development of an F-T process and in the improvement of the performance of the catalyst used in such processes.
The catalyst or catalyst precursor subjected to the ALTO treatment in accordance with this invention is characterized as the composite of a solid support, or powder component and a catalytic metal, or metals component inclusive of cobalt, all or a portion of which is present as metallic cobalt. It is obtained, e.g., by reduction of a precursor prepared by gellation, cogellation or impregnation techniques; e.g., precipitation of gels and cogels by the addition of a compound, or compounds of the catalytic metal, or metals, from solution as by addition of a base, or by the impregnation of a particulate solids support, or powder, with a solution containing a compound or salt of the catalytic metal, or metals. The treated composite is thus typically (i) a fresh catalyst precursor prepared by gellation, cogellation or impregnation techniques and reduced with a hydrogen containing gas or (ii) a catalyst used in a carbon monoxide hydrogenation reaction, comprising a substantial amount of reduced catalytic metal or metals; and the techniques used for obtaining such catalyst or catalyst precursors are well known to those skilled in the art. The catalytic precursor within the meaning of this invention is thus the harbinger composition which, when the reduced metal, or metals, is oxidized, by contact with an oxidant in the presence of water, and then reduced, as by contact with hydrogen, is comprised of sufficient of the dispersed reduced catalytic metal, or metals, that it is useful in catalyzing carbon monoxide hydrogenation reactions.
In the preparation of such catalyst precursor a metal, or metals, component, including particularly cobalt, catalytically active for conducting carbon monoxide hydrogenation reactions, especially F-T synthesis reactions, is composited with a particulate solids support, or powder, suitably a refractory inorganic oxide support, preferably a crystalline aluminosilicate zeolite, natural or synthetic, alumina, silica, silica-alumina, titania, or the like. For example, in impregnating a particulate support, or powder, the support or powder is contacted with a solution containing a salt, or compound, of cobalt; and if desired, an additional metal, or metals, preferably a Group VIIB or Group VIII metal, or metals, of the Periodic Table of the Elements (Sargent-Welch Scientific Company; Copyright 1968) to impregnate and deposit the metal, or metals, upon the powder or support. Exemplary of such metals are ruthenium, Re, Th, and the like, added to the cobalt to form the catalytically active metal component. Generally, from about 2 percent to about 70 percent, preferably from about 5 percent to about 25 percent metallic metal, or metals, inclusive of cobalt, is deposited upon the particulate solids support or powder, based upon the total weight (wt. %; dry basis) of the catalyst precursor (or finished catalyst). Catalysts having a relatively high metal, or metals, loading are preferred because these catalysts can be loaded into slurry bubble columns over a broad range of concentrations for activation, and use for conducting an F-T reactions up to that high concentration in which mixing and pumping the slurry becomes limiting. The impregnated powder or support is contacted with a reducing agent, suitably hydrogen at elevated temperature, to reduce the metal component to its lowest valence state, generally to metallic metal.
The catalyst precursor or catalyst, a composite constituted of a support component and a catalytic metal, or metals, component all or portion of which is in reduced metal form, is oxidized at low temperature by contact with water to which an oxidant has been added to form the oxidized catalyst precursor or catalyst. In conducting the ALTO treatment, at least the reduced cobalt metal, and preferably all of the reduced metals where there is an additional metal, or metals, are reacted to obtain the oxidation of all or a portion of said reduced metal or metals. The metal, or metals, in metallic state is (are) converted to cationic or oxo-anionic species. Unlike conventional oxidation with an oxygen containing gas as practiced, e.g., in R-O-R, the oxidation carried out in the presence of water is obtained at lower temperature, typically at temperatures lower than 200xc2x0 C., preferably lower than 150xc2x0 C., and most preferably lower than 100xc2x0 C. In contrast, in a conventional R-O-R treatment with an oxygen containing gas to fully oxidize the metals requires temperatures higher than 200xc2x0 C., and typically higher than 300xc2x0 C. if more refractory metal, e.g., rhenium, is present on the catalyst. For example, in an ALTO treatment of a Co/TiO2 catalyst or catalyst precursor slurred in water with an added oxidant converts, at least initially, all or a portion of the cobalt metal particles to Co ions.
In the ALTO treatment all or a portion of the cobalt, and all or a portion of an additional catalytic metal, or metals, of the metal, or metals component of the catalyst or catalyst precursor is oxidized to metal ions or metal oxy-anions, e.g., Co2+ or ReO4xe2x88x92. Temperatures ranging from about 0xc2x0 C. to about 200xc2x0 C., preferably from about 10xc2x0 C. to about 150xc2x0 C., and most preferably from about 15xc2x0 C. to about 120xc2x0 C. are used in the ALTO treating step. Preferred operating conditions are dependent upon the nature and concentration of the oxidant that has been added to the water. Pressure can be set to any value compatible with the design of the process. The amount of water added to the catalyst ranges from the incipient wetness volume to large excess up to 100 times the volume of treated catalyst. The oxidant may be added as a gaseous oxidant bubbling in the catalyst/water slurry. Typically, when a gaseous oxidant is bubbled into the liquid excess water is added, usually more than twice the volume of catalyst. Appropriate gaseous oxidants include, but are not limited to, oxygen containing gas, e.g., air, an ozone or nitrogen oxide containing gases. It may be preferable to operate at pressure higher than atmospheric pressure when the oxidant solubility in water is limited, e.g., when treating with air. In a preferred mode of operation, the oxidant is added as an aqueous solution of a soluble compound, which include, but are not limited to, nitrate derivatives, such as nitric acid, hydroperoxides and peroxides, e.g., hydrogen peroxide. Oxidants containing known poisons of the said catalyst or catalyst precursor are preferentially avoided, e.g., Cl, Br, P and S when the catalyst is employed in carbon monoxide hydrogenation process. The concentrations of oxidant vary from a low of 5 parts per million (ppm soluble O2 in water for example) to about 50 wt. %, preferably from about 10 ppm to about 30 wt. %. Contact times may vary from few seconds to about 24 hours. In general, preferred contact times depends upon the metal particle size, metal loading, the mode of operation and the concentration and nature of the oxidants. For example larger metal particle would require longer contact times because the oxidation of the inner core of the metal particle is generally diffusion limited. The mode of operation is also affected by the contact times. When a gaseous oxidant is bubbled into the catalyst/water slurry and the concentration dissolved in the water is low, longer contact times are required typically 2 hours to 48 hours, preferably 4 to 16 hours. Contacting times need to be at least sufficient to provide enough oxidant to obtain the desired level of oxidation of the metal particles. When water-soluble oxidants are used, the reaction rate depends upon the concentration and can be much faster. Typically, contact times vary from 15 seconds to 4 hours, preferably 1 to 60 minutes. The amount of soluble oxidant added is sufficient to provide the desired level of oxidation. Preferably, the amount of oxidant is sufficient to complete the oxidation of the metal or metals, and most preferably an excess of oxidant is used. The preferred mode of operation is the addition of the solution in incipient wetness conditions, which may be carried out in multiple steps. When excess water is used, the evaporation method is preferred to the filtration method, especially if the metals are oxidized to soluble salts such as oxo-anionic salts, e.g., perrhenates.
The ALTO process may be carried out in any suitable reactor, including but not limited to fixed bed or slurry reactors. For example, in a fixed bed operation, the oxidant/water mixture is added in a flow-through mode. Typically, the fixed bed may be flooded to fill the void volume of the bed or alternatively operated in trickle-flow regime. For an external operation associated with the use of a bubble column or a moving bed, the oxidation may be carried in a batch or continuous mode.
After the removal of excess water, the catalyst or catalyst precursor is preferably dried, and contacted with hydrogen, or a hydrogen containing gas, at elevated temperature, preferably at temperature ranging from about 200xc2x0 C. to about 600xc2x0 C., or preferably from about 300xc2x0 C. to about 450xc2x0 C., at hydrogen partial pressures ranging from about 0.1 atm to about 40 atm, sufficient to convert the metal compounds or salts to essentially the zero valent state of the metal thereof, i.e., metallic metal. Most preferably, prior to the reduction treatment, the catalyst or catalyst precursor may be further treated at high temperature in presence of an inert gas or oxygen containing gas. In conducting the reaction, temperature ranges generally from about 100xc2x0 C. to about 700xc2x0 C., preferably from about 200xc2x0 to about 600xc2x0 C. Thermal or calcination treatments commonly used in the decomposition of metal salts, particularly nitrates and hydroxides to form refractory oxides are appropriate.
The catalysts, or oxidized catalyst precursors after they have been reduced are used in a hydrogenation process, preferably a carbon monoxide hydrogenation process, particularly one wherein liquid, gaseous or solid hydrocarbon products are formed by contacting a syn gas comprising a mixture of H2 and CO with the F-T hydrocarbon conversion catalyst of this invention under water gas shifting or non-shifting conditions; but preferably non-shifting conditions in which little or no water gas shift reaction occurs, particularly when the catalytic metal comprises Co, particularly Re or Ru or mixtures of one or both of these metals with cobalt.
The hydrocarbons produced in the F-T hydrocarbon conversion process are typically upgraded to more valuable products by subjecting all or a portion of the C5+ hydrocarbons to fractionation and/or conversion. By xe2x80x9cconversionxe2x80x9d is meant one or more operations in which the molecular structure of at least a portion of the hydrocarbon is changed and includes both non-catalytic processing, e.g., steam cracking, and catalytic processing, e.g., catalytic cracking, in which the portion, or fraction, is contacted with a suitable catalyst. If hydrogen is present as a reactant, such process steps are typically referred to as hydroconversion and variously as hydroisomerization, hydrocracking, hydrodewaxing, hydrorefining and the like. More rigorous hydrorefining is typically referred to as hydrotreating. These reactions are conducted under conditions well documented in the literature for the hydroconversion of hydrocarbon feeds, including hydrocarbon feeds rich in paraffins. Illustrative, but non-limiting, examples of more valuable products from such feeds by these processes include synthetic crude oil, liquid fuel, emulsions, purified olefins, solvents, monomers or polymers, lubricant oils, medicinal oils, waxy hydrocarbons, various nitrogen- or oxygen-containing products and the like. Examples of liquid fuels includes gasoline, diesel fuel and jet fuel, while lubricating oil includes automotive oil, jet oil, turbine oil and the like. Industrial oils include well drilling fluids, agricultural oils, heat transfer oils and the like.